One-piece hollow camshafts and process for producing same

ABSTRACT

The invention relates to a hollow camshaft consisting of a metal or metal alloy or plastic which can be cold-formed and heat-treated and has a fiber flow parallel to the exterior contours of the camshaft, and to a method for producing it comprising the steps of: 
     Placing a pipe outlet element and filling it with fluid; 
     Sealing at least the pipe section to be widened; 
     Applying an internal high pressure suitable for widening the pipe section to produce an intermediate product pre-form, wherein, while the internal high pressure is applied, the hollow pipe is compressed against a movable die in the direction of its long axis in such a way that accumulations of material occur at approximately those places where it is intended to create cams; 
     Shaping the intermediate product pre-form into a shape corresponding to a final camshaft shape by means of the internal high pressure shaping method, so that cams are shaped at the desired locations; 
     If necessary, finishing the cams by heat-treatment.

This is a division of application Ser. No. 08/648,072, filed Jul. 12,1996 which is a 371 of PCT/DE94/01218, filed Nov. 26, 1994.

BACKGROUND OF THE INVENTION

The invention relates to a hollow camshaft as well as a process forproducing hollow camshafts by employing internal high pressure shapingof hollow pipes.

The invention in particular relates to processes employing pre-shapingin the course of internal high pressure shaping for designing completehollow shafts from pipe sections, wherein pipe sections are hereunderstood to be arbitrary long hollow bodies, i.e. also square pipes,hexagonal pipes or other hollow profiles.

In comparison with solid hollow shafts, hollow camshafts, such asschematically represented in FIG. 1, offer a weight advantage of up to75% over shafts of solid material. In comparison to this, constructedcamshafts such as known from DE-37 04 092, EP 278292, EP 278292,DE-3428372 or EP 290758 or EP 303845, only offer a weight advantage of40% in comparison with solid shafts. In addition, constructed camshaftsremain to be comparatively expensive to produce, because separate camsand hollow pipes must be stocked and then placed together into a die ina suitable manner. Thus, expensive individual components must be kept instock and combined.

For this reason hollow camshafts, although their constructed shape hasbeen described theoretically and also in patent applications, have notyet been used in motor vehicles up to now.

The reason for this lies in a suitable process technique for producingthese shafts. Suggestions for producing constructed hollow camshafts arerecited, for example, in DE-C-19 10 517. These process techniques are,however, completely unusable for producing hollow camshafts from pipesections. The required degrees of shaping can neither be achieved byelectromagnetic nor by electro-hydraulic shaping. Swaging or roundkneading as described in DE-C-37 36 453 also has not meet the goal. Thistype of producing hollow camshafts fails particularly because of theone-sided mass distribution in the cams. The one-step internal highpressure shaping by means of a tool in accordance with U.S. Pat. No.2,892,254 also does not meet the goal. Although axial feeding ofmaterial is provided with this mode of operation, filling the die at thecam tips is not achieved and a considerable stretching of the walls ofthe cams results. FIG. 2 shows the undesired stretching of the walls ona cam which was produced by means of the known one-step internal highpressure shaping method.

The internal high pressure method or IHV method is here understood tomean the method which was described for example in the IndustrieanzeigerIndustrial Journal! No. 20 of Mar. 9, 1984, or in "Materialumformtechik"Material Shaping Technology!, vol. 1D/91, pp. 15 et seq.: A. Ebbinghaus"Prazisionswerkstucke in Leichtbauweise, hergestellt durchInnenhochdruckumformen" Precision Workpieces in LightweightConstruction, Produced by Internal High Pressure Shaping!, or in"Werkstoff und Betrieb" Material and Factory! 123 (1990), 3, pp. 241 to243: A. Ebbinghaus "Wirtschaftliches Konstruieren mitinnenhochdruckgeformten Prazisionswerkstucken" Economical Constructionby Means of Precision Workpieces Produced by Internal High PressureShaping!, or "Werkstoff und Betrieb" 122, (1991), 11, (1989), pp. 933 to938. To avoid repetition, reference is made in full to their disclosurein what follows.

These processes have been used up to now for example for producingflanges, as described in EP-2395052, or for producing constructed hollowcamshafts for fastening cams on a pipe for producing hollow camshafts.

SUMMARY OF THE INVENTION

In contrast to this it is the object of the invention to provide hollowcamshaft which can be economically produced, and a process formanufacturing it.

This object is attained by means of a one-piece camshaft consisting ofmetal or plastic which can be cold-shaped and heat-treated and has aflow of fibers which is parallel with the outer contours of thecamshaft.

Carbon-containing steel, which can be sufficiently stretched, but canalso be heat-treated for hardening, can preferably be used as thematerial for this.

Suitable materials can be: 19MnB4, 16MnCr5, CK45, 19CrKo44, 15Cr3,C25-steel, X3NiCoMoTilB 9 5, β-C-titanium alloys and those showingsimilar properties.

The invention furthermore relates to a method for producing hollowshafts, employing the internal high pressure shaping of hollow pipes,distinguished by the following steps:

Placing a pipe outlet element and filling it with fluid;

Sealing at least the pipe section to be widened;

Applying an internal high pressure suitable for widening the pipesection to produce an intermediate product pre-form, wherein, while theinternal high pressure is applied, the hollow pipe is compressed againsta movable die in the direction of its long axis in such a way thataccumulations of material occur at approximately those places where itis intended to create cams;

Shaping the intermediate product pre-form into a shape corresponding toa final camshaft shape by means of the internal high pressure shapingmethod, so that cams are shaped at the desired locations; if necessary,finishing the cams by heat-treatment or the like.

Other advantages will be appreciated from the following disclosure andclaims.

Thus, proceeding beyond the known internal high pressure shaping method(see "Tagungsband des 14. umformtechnischen Kolloquiums" Minutes of the14th Colloquium on Shaping Technology! in Hannover 1993, to thedisclosure of which reference is made in full), in accordance with theinvention a special two-step shaping method is provided, by means ofwhich hollow camshafts can be economically produced. In the process,exactness of shape and low wall stretching at the cam tips is achievedin an appropriate internal high pressure shaping method.

In a surprising manner it is possible to produce camshafts havingessentially constant wall thicknesses.

The invention will be explained in detail below, making use of theattached drawings and the subsequent description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a camshaft in accordance withthe invention, produced in two shaping steps;

FIG. 2A is a schematic representation of a camshaft produced in a singleshaping step in accordance with the prior art;

FIG. 2B is a section along the line 2B--2B of FIG. 2A;

FIG. 3A is a section through a cam of a pre-form made in accordance withthe invention;

FIG. 3B is a section through the final form of the cam created from thepre-form of FIG. 3A;

FIG. 4A is a cross-sectional view of a cam pre-form after a firstshaping step;

FIG. 4B is a partial longitudinal sectional view of the cam preformshown in FIG. 4A;

FIG. 5A is a cross-sectional view of a second cam pre-form after a firstshaping step;

FIG. 6A is a cross-sectional view of a third cam pre-form after a firstshaping step;

FIG. 6B is a partial longitudinal sectional view of the cam preform inshown in FIG. 6A;

FIG. 7A is an enlarged cross-sectional view of a portion of the finalcam shown in FIG. 1, and, produced from the pre-forms shown in FIGS. 4to 6;

FIG. 7B is a partial longitudinal sectional view of the final cam shownin FIG. 7A;

FIG. 8 is a partial longitudinal sectional photograph of a final camshaft manufactured in accordance with the present invention andillustrating the parallel metal fiber flow.

A camshaft in accordance with the invention generally indicated at 10 inFIG. 1, is distinguished in that--in contrast to the conventionalconstructed hollow camshafts--it does not have attached cams, but isproduced from a single original pipe without any further individualelements in several shaping steps. As a result, an extremelyadvantageous fiber flow extending parallel with the outer contours ofthe shaft is achieved, which results in great sturdiness of the shaftand thus in reduced weight. If desired, it is possible at the same timeto set an essentially constant wall thickness of 2 mm to 5 mm, forexample, which has a tolerance of only approximately 0.5 mm. As shown inFIG. 1, a hollow tube has been expanded at spaced longitudinal locations62,64 to form the hollow camshaft in accordance with the invention.

No problems of a possible detachment of the cams from the central pipeoccur. It is important that a material is used which has sufficientcold-forming capability while at the same time has hardness or thecapability of being hardened in order to create wear-resistant cams.

This can be, for example, suitable carbon-containing steel which can beheat-treated but also is sufficiently stretchable for tolerating suchextensive shaping without tearing. Depending on the intended use of theshaft in stationary engines, aircraft engines, ship's engines or landvehicles, it is of course possible to employ other light and/or sturdymaterials, such as aluminum alloys, titanium alloys and other materialsfamiliar to one skilled in the art, such as 19MNB4, 16MnCr5, CK45,19CrKo44, 15Cr3, C25-steel, X3NiCoMoTilB 9 5, β-C-titanium alloys andthose showing similar properties. Depending on the intended use it ispossible to employ an expensive, light material--this is suggested forexample for aircraft engine construction--or a heavier material ifweight is not an essential value.

The method in accordance with the invention preferably provides at leasta two-step shaping process consisting of pre-form and finished form,wherein particular importance is assigned to the preform in the firstshaping step. So that exactness of shape and little wall stretching areassured to the finished form in the second shaping step, it is necessaryto provide an advantageous mass distribution of the cam wall in thepre-form. In each case the mass distribution can only be achieved if thematerial is being axially pushed longitudinally inwards on oppositessides of each cam by a movable die. The axial feeding at each cam isperformed by means of tool technology known per se.

FIG. 2B shows a section through a hollow camshaft formed by means of asingle shaping step in accordance with the prior art, and it can be seenparticularly from the cross section of the cam in FIG. 2B thatstretching of the metal in the one-step method is too high and thatconsiderable differences in wall thickness occur, namely a wallthickness of approximately 1.0 mm at the cam tip 12 and of 3.0 mm on thepipe wall 14 opposite the cam tip. Because of this, sturdiness is notassured.

In contrast to this, a cam as produced in accordance with the method ofthe present invention is first formed into a pre-form 16, as showngenerally in FIG. 3A It can be appreciated that the cam preformillustrated in FIG. 3A has relatively rounded tip ends 20 in comparisonwith the relatively pointed cam tip ends 22 in the final camshaft, asshown in FIG. 3B. Because initially a cam is produced into the pre-form16 which is wider, preferably approximately 10 to 40% wider andparticularly preferred approximately 20 to 30% wider than the finishedcamshaft form 18, as shown generally in FIG. 3B, and is also more roundthan the finished form 10, it is possible to provide in the preform 16 asufficient accumulation of material at the critical cam location 20where maximum material stretching takes place. This accumulation ofmaterial can then in a second shaping step be shaped into the finishedcam form 10 for example by means of an internal high pressure shapingmethod. However, if desired, other shaping methods, such as swaging, canalso be employed. It can be seen that for the finished camshaft 10, thedesired accumulation of material under the cam 22 can be achieved bymeans of two shaping steps.

In the embodiment FIG. 4A, for example, the cam pre-form 26 has roundcam tips R1 in order to prevent unnecessary material stretching. The campre-form width B1, as shown in FIG. 4B, is wider by 10 to 40%,preferably 20 to 30%, than the finish form width B2, as shown in FIG.7A. In the course of shaping the finished form, the round and readypre-form 26 is axially compressed in the second shaping step, resultingin the finished camshaft 10 illustrated in FIGS. 7A and 7B. Thiscompressing process prevents the excessive stretching of the pipe walland forces a high degree of shape accuracy, which makes late finishingunnecessary.

FIGS. 5A and 5B show another possible pre-form variant indicated at 32for the first shaping step, with a directedly increased circumference U1of the cams in the pre-form which is approximately 5 to 20% greater,preferably 10% greater than the finished form. In this case the materialflows in the circumferential direction U2 into the cam tip in the courseof producing the finished form. In this connection reference is made toFIGS. 6A and 6B showing a partially collapsed cam shape which then isshaped into a cam in a subsequent shaping step.

The finished cam shape is then represented in FIGS. 7A and 7B, and canbe produced from every one of the pre-forms of FIGS. 4A, 5A, and 6A. Inone embodiment--to which the invention is not limited in any way--theexterior cam height is 50 mm from the cam tip 40 to the cam base 42; thepipe section has a diameter of 30 mm and a wall thickness ofapproximately 3 mm, the cam tip 40 has a wall thickness of 2 to 3 mm andthe cam base 42 has a wall thickness of up to 5 mm, since the greatestaccumulation of material takes place there. The cam pre-forms of FIGS.4A, 5A, and 6A and finished forms represented in these figures (e.g.,FIG. 7A and 7B) are of course not limiting. If required, it is alsopossible to form bearing rings and the like on the camshaft 10 inaccordance with this principle.

FIG. 8 is a photographic representation of the fiber flow in a partialsection of the camshaft 10 of the invention from which it can be seenthat the fibers 50 extend parallel in respect to the walls of thecamshaft.

Finally, the possibility of a combination of internal high pressureshaping methods and other shaping methods should be mentioned a suitablepre-form can be produced, for example, by means of internal highpressure shaping and can be subsequently finished by swaging or othersuitable methods familiar to one skilled in the art.

The shaft can be further processed in accordance with the IHV method ina known manner, for example hardened. Useful for this are the knownprocesses, such as finishing of the shaft by means of one or severalfinishing methods selected from the group comprising nitrocarbonizing,plasma-nitriding, treating with boron, laser hardening, hardeningwithout carbonizing, induction hardening, flame hardening, electron-beamhardening, case hardening.

It is pointed out that the shafts in accordance with the invention canbe produced by means of the IHV method with the most varied cam angles,wall thicknesses, wall thickness ratios and are in no way limited to anyone of the described embodiments.

We claim:
 1. A method for producing a hollow camshaft comprising:i)forming a camshaft preform by:providing a hollow metal pipe; expanding adiameter of said hollow pipe at spaced longitudinal locations byapplying pressurized fluid internally to said hollow pipe; axiallycompressing said hollow pipe during said expanding step to accummulatemetal at said spaced longitudinal locations; and ii) forming a finalcamshaft from said camshaft preform by swaging.
 2. The method accordingto claim 1, wherein said predetermined widths of said cam preforms arebetween 10% to 40% greater than the widths of the cams of the finalcamshaft.
 3. The method according to claim 1, wherein said cam preformshave relatively rounded tip ends, and said method further comprises astep of forming relatively pointed cam tip ends from said relativelyrounded tip ends.
 4. The method according to claim 3, wherein said finalcamshaft is hardened using a method selected from the group comprising:nitrocarbonizing, plasma-nitriding, boron treatment, laser hardening,induction hardening, heat treatment, electron beam hardening and casehardening.
 5. The method according to claim 1, wherein saidpredetermined widths of said cam preforms are between 20% to 30% greaterthan the widths of cams of said final camshaft.
 6. The method accordingto claim 1, wherein cams of said final camshaft have relatively smallerwidths in the longitudinal direction than widths of the cam preforms. 7.The method according to claim 1, wherein cams of said final camshafthave a relatively smaller circumference than a circumference of the campreforms.